-- A program similar to the Echo program, written
-- without monads as the first step to Fudgets adoption.
-- The program also demonstrates use of the DOM Level1 framework
-- also implemented in CPS style.
module EchoCPS where
-- This module contains useful functions to access properties of underlying
-- This module contains functions to wrap expressions in CPS style
-- This module contains Roman-Decimal conversion funcitons
-- The modules under DOM are autogenerated from IDL
-- The modules under CDOM are utilities to simplify the use of DOM facilities
-- This module contains functions to obtain timestamps
-- Line-mode output: create a <div> element, place a text in it,
-- append to the parent element (document body). The mbb argument
-- specifies whether the text will be inserted before a specific
-- element (Just) or just appended to the end of the document (Nothing).
-- The s argument is the string to output, and the c argument is
-- the continuation.
putLine s mbb c = getHTMLDocument $ \doc ->
documentBody doc $ \body ->
-- body contains reference to the HTML document <body> element
-- which is the parent of all <div>'s displaying the lines being output
mkDiv doc $ \dv ->
mkText doc s $ \tx ->
addChild tx dv $ \ch ->
-- Functions whose names start with mk (mkDiv, mkText) are autogenerated
-- wrappers over DOM methods which create elements bearing appropriate
-- HTML tags. Above, a <div> and a #text elements were created, and the #text
-- element was added to the <div> as a child.
let iac = case mbb of
Nothing -> addChild dv
Just b -> insertChild b dv
-- Based on the mbb argument, decision was made whether to add the child
-- (previously created <div>) to the end of the document (addChild), or to
-- insert the <div> before the specific element (insertChild).
in iac body $ \ct ->
main = getHTMLDocument $ \doc ->
documentBody doc $ \body ->
-- The same sequence as above, to get the reference to the
-- HTML document's <body> element.
mkInput doc $ \inp ->
addChild inp body $ \_ ->
-- The <input> element was created and added to the <body> element
set'id "input-echo" inp $ \_ ->
-- The set' family of methods deal with setting nodes' properties.
-- The above expression sets the id attribute of the <input> element
-- to "input-echo", so in plain HTML this would be:
-- <input id="input-echo">
set'on "keypress" (inkey inp) inp $ \_ ->
-- The set'on "keypress" is same as specifying
focus inp $ id
-- The focus is a DOM method setting input focus on the element.
-- The id call at the very end "closes" the CPS chain of function calls.
-- A function to convert between Roman and Decimal presentations.
-- It also features exception handling that prevents the program
-- from crash if something wrong is entered. Input in error will be
-- converted into an empty string. The fromRoman and toRoman functions
-- call error in the situation when input cannot be processed.
romdec :: String -> (String, String)
romdec v =
let rom = (catchJS ((show . fromRoman) v) (\_ -> ""))
dec = (catchJS ((toRoman . read) v) (\_ -> ""))
in (rom, dec)
-- The "onkeypress" handler: it does all the job. Note the o argument:
-- it holds reference to the element a handler is attached to. This forms
-- a closure (which may not be very much desired for some browsers), but
-- makes it extremely easy to distinguish between elements that cause
-- the handler to fire.
inkey :: THTMLInputElement -> a -> Bool
inkey o e = unsafeGetProperty "keyCode" e $ \kcs ->
unsafeToNum kcs $ \kci ->
-- Since there is no completed framework on event properties, we have
-- to use the unsafe interface. The two lines above extract the "keyCode"
-- property from the event received, and gets the numeric value of it.
-- new Number (e.keyCode)
if kci == 13
-- We are only interested in the Enter key whose code is 13.
-- Other keys just edit the value in the <input> element, and do not
-- need to be handled.
get'value o $ \val ->
unsafeToString val $ \v ->
-- The get' family of methods retrieve properties from nodes (elements).
-- They are basically wrappers over unsafeGetProperty, but IDL definition
-- guarantee that properties not defined per DOM are not accessible through
-- these functions. But we still need to obtain string value of it.
if length v > 0
-- If it was not just pressing Enter on an empty input box.
getTimeStamp $ \t1 ->
-- Obtain the current time stamp
toCPE (romdec v) $ \(rom, dec) ->
-- toCPE wraps an expression in the CPS style expression. See above,
-- the romdec function returns a tuple of two conversion results.
rom `seq` dec `seq` getTimeStamp $ \t2 ->
-- Usage of seq's is necessary because seq on the tuple forced by toCPE
-- does not evaluate deep into the structure: only to WHNF.
-- We want to measure the Roman-Deciman conversion time. So we have to force
-- the members of the tuple to evaluate before we get the next time stamp.
putLine (v ++ " " ++ rom ++ " " ++ dec ++ " " ++ show (t2 - t1) ++ " ms")
(Just o) $ \_ ->
-- putLine places the formatted output string before the <input> element
-- (Just o). So, as the program works, lines of output start at the top
-- of the browser window, and the <input> element moves downwards as new
-- output apprears.
set'value "" o $ \_ ->
-- Reset the <input> element, erase what was typed in.
-- The final lambda returns True regardless, which means that
-- the browser should take the default action on the user's input
-- (keys pressed).